Agree with Wikipedia. The possible effects of glucosamine sulfate in patients with osteoarthritis may be the result of its anti-inflammatory activity,  the stimulation of the synthesis of proteoglycans,  and the decrease in catabolic activity of chondrocytes inhibiting the synthesis of proteolytic enzymes and other substances that contribute to damage cartilage matrix and cause death of articular chondrocytes.
Not much. Glucosamine benefits are hard to scientifically measure. It seems to help joint pain in about 50% of people who take it. The dose level is unclear and the purity of commercial preparations is unclear as well. The proper dose has not been worked out. So the use of glucosamine is still murky scientifically.
Rebuild cartilage. The combination of glucosamine and chondroitin has been shown in studies to (1) provide a degree of pain relief in the short term, and (2) trigger growth of new cartilage when taken for at least two years straight. So it can help you pain and reverse some of the chronic damage. You can find this in the health food store. MSM is not necessary and adds to the cost of the product.